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Gold resources of Victoria
... In crust under triaxial stress (principal compressive stresses a,>a 2>a3 ) , effective normal stresses are approximated by: in anticlinal closures predominate. Saddle reefs stacked along anticlinal hinge-planes were mined in some cases to depths approaching 1100 m (Baragwanath, 1929;Cox et al., 1991;Willman & Wilkinson, 1992). In longitudinal section, individual saddle reefs can be traced along fold hinges for over 3 km continuous across one major and at least two minor culminations (Fig. 3b). ...
... Saddle reef vein system from the Bendigo goldfield, Victoria, Australia: (a) terminology of typical saddle reefs (hydrothermal infill in black); (b) W-E cross-strike section through the Great Extended Hustler's Mine main shaft, Bendigo, plus S-N longitudinal profile showing the continuity of saddle reefs through a portion of the goldfield (bolder lines denote most productive reefs) (afterBaragwanath, 1929). ...
Evidence exists in both active and ancient fold-thrust belts for extensive hydraulic connectivity over distances of kilometres to tens of kilometres along-strike during fold growth. Components of 3D structural permeability contributing to along-strike flow include fold closures with interlayered high and low permeability beds, continually resheared bedding planes, saddle reef structures with incrementally created hinge-line gapes, and various thrust flat-ramp and fault-fracture intersections. Structurally controlled directional permeability may arise both in 'normally' pressured crust (i.e. where fluid pressures are approximately hydrostatic) and at depth in strongly overpressured submetamorphic environments where fluid pressures approach lithostatic. Along-strike flow parallel to fold hinges is particularly likely where non-uniform shortening occurs along a fold-thrust belt during compressional inversion or continental collision. Interpretation of fluid redistribution from cross-strike sections coupled with 2D numerical modelling may then be highly misleading.
... (C) Stacked saddle reefs (red) in workings of the Hustlers mine, Bendigo, cross section looking north (modified from Baragwanath 1930). The post-ore lamprophyre dyke (blue) is Jurassic (159±6 Ma K-Ar; Cherry and Wilkinson 1994). ...
These explanatory notes accompany 38 color slides illustrating the structural setting of syn- and post-orogenic vein deposits in the Rhenish Massif, Harz and Ruhr coal district, part of the Variscan slate belt in Germany. Both this text and the slide presentation (in tablet 4:3 format and Adobe 1998 color) are available free of charge as Adobe pdf-files from the SGA website (www.e-sga.org). They are designed as open access teaching tools for projection and the study on-screen. This text explains each slide step-by-step.
Four well-documented deposits were selected as structural type localities : (1) the Eisenberg gold deposit, bedding-parallel veins in an anticline; (2) the Ramsbeck Zn-Pb-Ag deposit, veins controlled by the axial plane cleavage and reverse faults in an anticline; (3) the Auguste Victoria Zn-Pb-Ag deposit, breccia ore in a normal fault displacing the folds; and (4) the Clausthal Zellerfeld and Bad Grund Zn-Pb-Ag deposits in the Upper Harz, banded and breccia veins in wrench faults displacing the regional folds.
Current evidence suggests that most of Victoria is underlain by a relatively thick (20 km +) basement of sialic composition of assumed Proterozoic age. This basement is nowhere exposed and its structural relationship with exposed Palaeozoic rocks is conjectural. This uncertainty has resulted in both ensimatic and ensialic tectonic models being proposed for Victoria during the Cambrian. Mineralization associated with Cambrian igneous activity shows a variety of styles from minor orthomagmatic chromite deposits, through Au and Cu deposits of syngenetic or epigenetic origin, to FeMn, Ba occurrences of exhalative volcanogenic affiliation. Cambrian volcanism and associated sedimentation was followed by the deposition of dominantly quartz-rich turbidites with interbedded shale and siliceous units. Subsequent to the epi-Ordovician Benambran Orogeny, late Silurian crustal extension caused several rifts to open along roughly orthogonal NW and NE aligned fractures. Within these fault-bounded depressions, thick acid volcanic sequences were deposited in close association with shallow-marine sediments. Mineralization in these Upper Silurian rocks comprises polymetallic base-metal sulphide lenses and minor disseminations, at least some of which are of exhalative volcanogenic affiliation. The Silurian rifts were obliterated and their rocks strongly deformed during the Bindian (Bowning) deformation during late Silurian to early Devonian time. This in turn was followed by another episode of crustal extension and rifting, during which the formation of a broad meridional trough marks the Buchan Rift. A very thick sequence of largely subaerial bimodal volcanics is overlain by shelf limestone and mudstone. A variety of minor base metal, barite, manganese, and iron mineralization is hosted by these volcanics and shelf sediments. The mid-Devonian Tabberabberan Orogeny was followed in the Late Devonian by bimodal volcanism and granite intrusion, and "red-bed"-type non-marine sedimentation. In Central Victoria, thick bimodal volcanics were erupted into a series of cauldron subsidences and intruded by comagmatic granites. Bimodal volcanism also occurred in the Mount Howitt Province farther east, but was followed by deposition of extensive fluviatile and lacustrine sediments (mainly mudstone, sandstone, and minor conglomerate). In the Mansfield Basin, these contain minor sedimentary copper occurrences. There are four distinct episodes of granite emplacement in Victoria, namely Late Cambrian -Early Ordovician (Delamerian) in the Glenelg Zone; Early Silurian (Benambran) in the Highlands Zone; Early Devonian (Bindian) in the Grampians, Ararat-Bendigo, Highlands, and Mallacoota Zones; and Middle Devonian-Carboniferous (post Tabberabberan) in the Ararat- Bendigo, Melbourne, Howqua, and Highlands Zones. Data for the Delamerian granitoids are sketchy, but in the remaining groups S-type granitoids predominate with the exception of eastern Victoria, east of the Yalmy Fault (I-S line), where only I- and A-type granitoids occur. A variety of Sn, Mo, W deposits and prospects are associated with the Benambran and younger intrusive phases. Victoria is a major gold province which has produced nearly 2.5 × 106 kg gold. Primary gold occurs in a number of geological settings including veins and disseminations spatially associated with mafic Cambrian volcanism, vein deposits in turbiditic sequences of central and eastern Victoria, veins associated with mafic and intermediate intrusives of Mid to Late Devonian age, and minor amounts associated with a variety of granitoids and porphyry dykes.
Central Victoria, in S.E. Australia, is regarded as the classical area for gold mineralization associated with saddle reefs, and with reverse faults, within a turbidite terrane. However, there is also an important contribution by steep, brittle shear zones to the mineralization and two such areas, Fosterville and Heathcote are the subject of this article. The shear zones hosting mineralization are sinistral transpressional structures that, while accommodating regional shortening, do it by both crustal thickening and lateral expulsion. The mineralization occurs at dilational jogs or bends within the zones. The shearing and mineralization postdate the main phase of folding and thrusting but predate widespread post-tectonic granitoid intrusion. It is postulated that the shears are a part of the initial phases of orogenic collapse which culminated in the emplacement of the granites, as well as sedimentation and acid to bimodal vulcanicity. Furthermore, it is argued that mineralization within the saddle reefs and reverse faults that characterize Central Victoria are also related to the initial phase of orogenic collapse. A final postulate is that orogenic collapse provides a common tectonic thread to some turbidite hosted, epithermal and Archean lode types of gold mineralization.
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